It's a Half-Life 2 mod, where you play (as I'm sure you can guess) a pirate, a viking, or a knight. It's in beta, so there's a number of problems and it's nowhere near complete, but it's free, and it's pretty damn fun.
A lot of servers are infested with suicide-keggers (one of the pirate weapons happens to be a massive keg of gunpowder that obliterates anything in its vicinity) but the word is that it's getting nerfed in the next patch. However, I can't say I haven't had my fair share of fun with kegs either.
If the name alone isn't enough to interest you, chances are you're not going to like it. But if you're like me, and the thought of pirates, vikings, and knights murdering one another sounds badass to you, check it out.

